Chronesthesia is known as 'mental time travel,' and it allows us to think about the past or the future. Although vital to everyday life, it's seldom been studied. What parts of the brain light up when you get nostalgic?

Many people are familiar with the term synesthesia, when the brain takes physical sensual reality and swaps it out for a different sensation — for example, seeing sounds as colors. Chronesthesia is its more common, but lesser known temporal equivalent. Instead of being stuck in time, the way we are physically, we can remember the past or plan for the future. It's not seen as a big deal, but it is taking the mind away from real stimulus and letting it see something else.

For a time, researchers thought of this as just any other part of a world building. Humans have the capacity for building mental pictures of scenes that don't exist in reality. We can plot, scheme, dream, and use our imaginations. Whether we were doing it while thinking of the past, the present, or the future was all the same. Or so people thought.

In fact, we use an entirely different part of the brain when we're thinking about a different time. People were asked to think about taking the same walk in the past, the present, or the future. They were asked to think about taking the walk in their real life, and in an imagined past, present or future. Using an MRI, researchers saw that a different section of the brain lights up when people think about the past or future than when they think about the present. People aren't just building a picture and thinking, "Oh, this happened five years ago." They're relegating times other than the present to different portions of the mind.